Sulphide deposits have dominated production of primary zinc. Current estimates are that approximately 95% of the zinc mined worldwide is mined from sulfidic ore deposits, in which the ore is typically mixed with sulfides of copper, lead and/or iron. After grinding the ore, froth flotation is used to separate minerals from the gangue, taking advantage of differences in their hydrophobicity. (The concentrate is either smelted or undergoes roasting (to produce zinc oxide and sulfur dioxide), acid leaching (typically using at least some of the sulfur dioxide to produce acid) then electrowinning to recover zinc.
Unlike sulfides, zinc oxide is not readily separated from gangue materials by gravimetric means. Accordingly, the zinc oxide component of a mixed zinc oxide-zinc sulfide ore is typically discarded. However, increasing zinc demand has resulted in increased interest in recovering zinc from non-sulfide zinc ores, such as oxides and carbonate ores.
As far as the inventors are aware, the only commercial hydrometallurgical process for the production of zinc from zinc oxides is Skorpion Zinc, in Namibia. However, Skorpion has a minimum reported headgrade of 11% Zinc. Ores where the zinc oxide content is lower, or where there is a mixture of sulphide and oxide components, cannot presently be economically processed.
Carbonate based zinc ores are significant ore resources in terms of volume but cannot presently be processed. Zinc may be present in carbonate ores as either a pure zinc carbonate (the mineral smithsonite) or as a constituent of another mineral, such as iron carbonate (the mineral siderite, which may contain up to half the iron replaced by zinc). Pure zinc carbonate is uncommon and typically does not form economic deposits unless other zinc-bearing minerals are also present. There are a number of zinc-bearing siderite deposits known but these are generally regarded as untreatable due to the uneconomically high consumption of reagents necessary to achieve acceptable zinc dissolution.
The method of leaching of the present invention has as one object thereof to overcome the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.